The present invention relates to an address resolution system for an NBMA (Non-Broadcast, Multi-Access) network and using an NHRP (NBMA Next Hop Resolution Protocol) and, more particularly, to an address resolution system in which the NHRP and a conventional ATMARP (Asynchronous Transfer Mode Address Resolution Protocol) are interoperable.
The NHRP is discussed in IETF (Internet Engineering Task Force) as an address resolution protocol for the NBMA network. The specifications of the NHRP are described in, e.g., draft-ietf-rolc-nhrp-08.txt. In the following description, the NBMA network and its upper protocol are respectively assumed to be, but not limited to, an ATM (Asynchronous Transfer Mode) network and an IP (Internet Protocol).
Generally, to hold IP communication on the ATM network, means for obtaining an ATM address from the IP address of the other terminal is necessary. For this purpose, the NHRP scheme assigns the control of the IP address and ATM Address pairs of ATM terminals connected to the ATM network to distributed NHRP servers (NHSs) each being located at a particular area, e.g., LIS (Logical IP Subnetwork).
Assume that an ATM terminal intends to solve an ATM address for the EP address of another ATM terminal to communicate with, it sends an NHRP resolution request packet to a preselected NHS. If the NHS which received the request packet can resolve the address, it returns an NHRP resolution reply packet to the above ATM terminal. If otherwise, the NHS transfers the request packet to another NHS which seems to be controlling the IP address to be resolved. As a result, the request packet is sequentially transferred between a plurality of NHSs until it reaches an NHS capable of resolving the address.
Therefore, even if the other ATM terminal belongs to an LIS different from the LIS of the ATM terminal intending to solve the ATM address, the ATM address can be solved so long as the other ATM terminal is directly connected to the ATM network. If the other ATM terminal is not directly connected to the ATM network, there can be resolved the ATM address of a router (gateway) located at the outlet of the ATM network.
The ATMARP is another address resolution protocol for the ATM network and already standardized by IETF Requests for Comments (RFC). In the case of the ATMARP, address resolution is limited to the same LIS, and an ATMARP request packet is not transferred between different ATMARP servers.
The NHRP and ATMARP are entirely different protocols and have not heretofore been provided with interoperability.
When an LIS implemented with ATMARP adopts the NHRP as a new address resolution protocol, all the terminals belonging to the LIS must be replaced with terminals adaptive to the NHRP because the conventional NHRP servers cannot deal with the ATMARP. This prevents existing terminals using the ATMARP and terminals newly using the NHRP from existing together in the same LIS.
A simple solution to the above problem may be to have an NHRP server to play the role of an ATMARP server at the same time, i.e., causing an NHRP server and an ATMARP server to share the address information of NHRP terminals and ATMARP terminals. This, however, brings about the following problems. Even when a given NHRP terminal intends to resolve the address of an ATMARP terminal belonging to an LIS different from the LIS of the NHRP terminal, the ATM address of the ATMARP terminal itself can be resolved. Then, the NHRP terminals sets up SVC (Switched Virtual Connection) to the ATMARP terminal and then sends an IP packet via the SVC. At this instant, assume that the ATMARP terminal whose address should be resolved intends to send an IP packet to the NHRP terminal. Then, the ATMARP terminal resolves the ATM address of a router, or IP Next Hop, on the route to the NHRP terminal, sets up SVC to the router, and then sends an IP packet. That is, a different SVC is set up for each of transmission and receipt between the ATMARP terminal and the NHRP terminal, simply wasting the SVC set up by the NHRP terminal.
Moreover, the ATMARP terminal may expect that an IP packet from a different LIS arrives at the ATMARP terminal by way of the packet filtering function of a router belonging to the same LIS. However, because the NHRP terminal directly sets up an SVC to the ATMARP terminal, as stated above, the filtering function at the IP level available with the router cannot be used.